This inventive method relates generally to the field of prospecting for hydrocarbons, and more particularly, to extraction and analysis of compounds adsorbed to the surfaces and present in the pore spaces of samples such as drill cuttings and drill cores.
Fluid inclusion stratigraphy (xe2x80x9cFISxe2x80x9d) analysis methods have been known for more than ten years. The fluids that are analyzed in FIS analysis are trapped in tiny sealed enclosures in a sedimentary rock sample, and require some sort of physical deformation of the sample to release them. Subjecting the sample to a vacuum will not cause FIS gases to be released. The physical deformation is, most commonly, a mechanical crush or squeeze of the sample. Alternatively, a laser, an ion beam, or a tiny drill bit may be used to rupture at least one of the fluid-enclosing pockets in the sample.
In a typical FIS analysis, a sedimentary rock sample is crushed under vacuum and the trapped fluids that are released by the crush are analyzed, often with a mass spectrometer. When the sample or samples in the crush chamber are replaced, the chamber is pumped down again to the desired vacuum before crushing the new sample. This evacuation is necessary both to reduce background from the atmosphere and the previous sample and in order not to damage the mass spectrometer. In addition, the evacuation tends to pull out fluids trapped in the pore spaces or adsorbed onto grain surfaces of the new sample(s). These adsorbed and pore space gases are probably of origin different from that of the FIS gases which require crushing or squeezing to be released, and hence are considered a contaminant in FIS analysis that is either to be pumped off before the analysis begins or is subtracted as background from FIS results. The trapped FIS fluids (mostly gases when released under high vacuum) may be of ancient origin, which helps analysts understand formation and evolution of the subterranean formation. Equally or sometimes more usefully, the FIS results often exhibit anomalies that correspond to current hydrocarbon-bearing formations when the analysis is performed on rock chips obtained from well drilling or on outcrop samples. Either way, FIS analysis is useful in exploration and production of hydrocarbons.
Various traditional methods are the alternatives to FIS analysis to evaluate formation fluids from evidence obtained from well drilling or core samples. Some methods do this indirectly by providing estimates of pertinent rock properties. These methods include a variety of wire-line logging tools and formation testers. Porosity is either measured in core samples or more commonly estimated from logging tools using density, nuclear and acoustic properties. Permeability is estimated from core analysis or from nuclear magnetic resonance measurements. Formation fluid type (oil, gas, or water) is predicted from electrical resistivity measurements combined with other measurements from logging tools. Such indirect techniques can have limited reliability, which may lead to ambiguities in formation evaluation.
More direct techniques to evaluate rock properties and formation fluids while drilling also exist. These methods go by the general name of mud logging. Mud loggers describe the rock cuttings during drilling, use ultraviolet light to look for petroleum fluorescence, and monitor gas chromatographs to detect hydrocarbon gases from methane to pentane within drilling fluids. These more direct types of hydrocarbon detection also give variable results.
FIS analysis gives a fundamentally different type of information that is often needed to resolve uncertainties and ambiguities that arise from the above-identified traditional methods. Valuable as FIS analysis is, it suffers from the inherent drawback that what is analyzed may date back to distant, earlier times and may not accurately represent current formation conditions. It would be desirable to have a method that extracts more currently relevant information in statistically significant quantity. The present invention satisfies this need.
In one embodiment, the present invention is a method for petroleum exploration comprising the steps of obtaining one or more samples, which might be drilling cuttings or core or outcrop samples, from known surface or underground locations, then placing each sample under vacuum in the presence of a detector such as a mass spectrometer, using the mass spectrometer to analyze the composition and concentration of fluids released from interstitial cavities and pore spaces of the sample and also from surface adsorption, and predicting the presence and location of petroleum based on the measured concentration of petroleum indicator molecules.
In another embodiment, the present inventive method can be used to estimate rock properties such as permeability, the method comprising the steps of (a) placing a rock sample in an air-tight chamber connected to a vacuum pump and to a detector such as a mass spectrometer; (b) using the detector to measure the detection rate (ion current in the case of a mass spectrometer) as a function of elapsed time for at least one molecular constituent of the adsorbed and interstitial fluids released by the sample due to the reduced pressure and (c) comparing the response vs. time data from the unknown sample to similar data from samples with known values of the rock property, thereby estimating the rock property for the unknown sample.
In other embodiments, the present inventive method can be used to measure oil quality, or the location of the oil-water interface in a reservoir, by comparing measured concentrations of selected petroleum or non-petroleum constituent molecules.